FIG. 7 is a diagram of a related art high-frequency heating device (magnetron) (refer to Patent Reference 1). In FIG. 7, the AC power of a commercial power source 113 is waveform-shaped into a unilateral power by a rectifier filter 101 composed of a diode bridge 134 for rectifying the full waves of an AC waveform and a low-pass filter formed by a choke coil 119 and a smoothing capacitor 120. The unilateral power is converted to a high-frequency power of 20 to 50 kHz by an inverter 102 including a resonant circuit constituting a tank circuit with the inductance components of a resonance capacitor 121 and a transformer 107 and switching elements such as a power transistor 125 and a flywheel diode 122 serially connected to the resonance circuit. The high-frequency power generated on the primary side of the boosting transformer 107 is boosted by the boosting transformer 107 to generate a high-voltage high-frequency power on the secondary side. A circuit connected to the secondary side of the boosting transformer 107 is a high-voltage circuit 104 of a half-wave voltage doubler rectification system composed of a high-voltage capacitor 126 and a high-voltage diode 127. The high-voltage circuit 104 applies a high DC voltage (for example −4 kV) across the anode and cathode of a magnetron 106. Power is supplied from another secondary wiring 128 of the boosting transformer 107 to the heater of the magnetron 106 thus heating the cathode and causing electrons to reach the anode. This irradiates microwave energy onto an object to be heated in an oven chamber.
An inverter control circuit 103, receiving a setting output command Vref signal from a control panel 108, uses PWM control to vary On/Off of the power transistor 125 of the switching element to control supply of electric power to the secondary side thus controlling the strength of the microwave output from the magnetron. Blocks 101, 102, 103 and 104 surrounded by dotted lines are formed into an inverter circuit board 105 as a single unit by arranging a plurality of components on a printed circuit board. The interface between the inverter circuit board 105 and peripheral components is coupled at the connection parts CN1 to CN4 (numerals 109 to 112).
For the operation in the inverter control circuit 103 and PWM control, the earth of the high-voltage circuit 104 is connected to a chassis potential via an anode current resistor 135 composed of a resistor group and a connection part 109. The anode current of the magnetron 106 flows therein. The product of the anode current and the voltage applied across the anode and cathode of the magnetron 106 is the power inputted to the magnetron 106. With this configuration, it is possible to measure the value of the anode current once the voltage drop Via in the anode current sensing resistor 135 is detected. It is thus possible to convert a current to a voltage using a low-cost fixed resistor rather than using an expensive insulating type current transformer, thereby implementing an extremely economical current detector.
An anode current of several hundreds of milliamperes flows through the sensing resistor 135. The number of resistors connected in parallel (for example, resistors 142 to 144) and a constant should be determined so that the power loss of the resistor will fall within the rating and that the generated voltage will be easily handled by a circuit in the subsequent stage. The Via signal detected by the anode current sensing resistor 135 is inputted to a negative feedback controller 136. The deviation from the Vref signal coming from the control panel 108 is calculated and negative-feedback amplification is made to control the PWM output of the inverter 102 via a driving control amplifier circuit 168, thereby performing negative-feedback control of the magnetron 106 and making control to keep constant the anode current (refer to Patent Reference 1).
However, with the related art magnetron driving power source shown in FIG. 7, in case a fault should take place where the sensing resistor 135 is placed in the open mode (earth floating state) due to some cause such as breakage by extraneous electromagnetic wave energy, breakage under severe environment and mixing of faulty components, the high voltage of −4 kV or the like in the voltage doubler rectifier circuit 104 could be induced also into the control panel 108 operated by the user with their hands, thus causing a risk of an electric shock to the user. As a means for avoiding this risk, the high-frequency heating device shown in FIG. 8 arranges a protective capacitor 219 parallel to the sensing resistor 216 for detecting the anode current of the magnetron. The protective capacitor 219 is designed to have a larger capacitance value than that of a high-voltage capacitor 212 or a through-capacitor (not shown) while the sensing resistor 216 is in the open mode. With the operation of the protective capacitor 219, the high voltage is divided by the high-voltage capacitor 212, the through-capacitor and the protective capacitor 219, and the protective capacitor 219 is maintained at a low voltage value or at a low potential close to zero potential, which provides safety. This prevents a control panel circuit board 218 from floating at a high voltage even in the presence of open failure of the sensing resistor 216 thus assuring a safe configuration.
While the half-wave voltage doubler rectifier circuit has been described, it is also possible to provide safety to a full-wave voltage doubler rectifier circuit by way of the totally same configuration (refer to Patent Reference 2).
In the microwave oven shown in FIG. 9, in the event of a wire break in a conductor pattern 319a or 319b on an inverter circuit board 312 to which anode current sensing resistors 318a to 318d are connected, the resistance value of the anode current sensing resistor 318 increases and a drop in the voltage caused by an anode current increases. This leads to a higher level of the anode current sensing signal inputted to a control panel 322. Thus, the microwave oven is designed to detect a wire break and shut down the inverter operating when the level has risen abnormally thus preventing generation of sparks in a wire break section of the conductor pattern 319a or 319b. This reliably prevents burning or an electric shock caused by sparks (refer to Patent Reference 3).    Patent Reference 1: JP-A-10-172749    Patent Reference 2: JP-A-10-284245    Patent Reference 3: JP-A-2001-15260